Exterior projects
Siding types and how much more paint rough surfaces take
The same house takes 9 gallons in smooth panels and over 13 in painted brick. Here is where the extra goes, which product each surface needs, and how to plan for it.
Why the siding matters
Exterior paint labels quote coverage on a smooth surface, usually 350 to 400 sq ft per gallon. Very little of a house is smooth. Lap siding has board edges and drip lines; rough-sawn cedar has a fibrous surface that drinks paint; stucco and brick are textured, porous and full of hollows. Each adds surface to wet and paint to fill, and on masonry the first coat soaks in. Choosing the right product for the surface matters as much as the quantity: an ordinary house paint on fresh stucco, or a thin coat of elastomeric, fails early.
The siding factors
We multiply the measured wall area by a factor for the siding before dividing by coverage:
| Siding | Factor | What adds the surface |
|---|---|---|
| Smooth: flat panels, smooth fibre cement, metal | 1.0 | Nothing much |
| Lap or clapboard | 1.1 | Board edges and the underside of each lap |
| Rough-sawn wood, shingles, shakes | 1.3 | Fibrous surface and edges that absorb paint |
| Stucco | 1.4 | Texture and porosity |
| Brick | 1.5 | Mortar joints, texture and high absorption |
The factors are site estimates based on how much lower the manufacturers’ coverage ranges are for rough and textured surfaces than for smooth ones; they are applied in the exterior house paint calculator and listed in the coverage rates reference. If your paint’s data sheet gives a coverage for your surface, use it with a factor of 1.0 instead.
One house, five sidings
A one-storey ranch in each siding
1,490 sq ft of net wall (160 ft perimeter, 9 ft walls, two gables, two doors, ten windows), two coats at 350 sq ft per gallon, 5% allowance.
- Smooth panels, × 1.0: 8.94 gallons, buy a 5-gallon pail and 4 gallons
- Lap siding, × 1.1: 9.83 gallons, buy two 5-gallon pails
- Rough-sawn wood, × 1.3: 11.62 gallons
- Stucco, × 1.4: 12.52 gallons
- Painted brick, × 1.5: 13.41 gallons
From the exterior house paint calculator. The difference between the smoothest and the roughest surface is four and a half gallons on a modest house.
Wood siding
Lap and clapboard are the standard case. Paint the bottom edge of each board, which is where water collects and where paint fails first. A brush works the paint into the lap better than a roller; many painters spray and back-brush.
Rough-sawn wood, shingles and shakes absorb more, especially when weathered. A solid-colour stain is often a better choice than paint on rough wood, because it soaks in and does not peel; its coverage is lower than paint’s. Cedar and redwood contain tannins that bleed through light paint as brown streaks unless a stain-blocking primer goes on first, as the primer guide explains.
Stucco
Stucco is textured, porous and alkaline when new. New stucco needs time to cure before painting, commonly around a month, and then a masonry primer or conditioner that tolerates the alkalinity. Two products are common:
| Product | Smooth | Medium texture | Heavy texture | What it is for |
|---|---|---|---|---|
| Acrylic masonry paint | 250 sq ft/gal | 200 | 150 | Colour and protection on sound stucco |
| Elastomeric coating | 100 | 80 | 65 | Thick, flexible film that bridges hairline cracks |
930 sq ft of medium-texture stucco
- Acrylic masonry paint, two coats at 200 sq ft per gallon: 9.77 gallons, plus about 5 gallons of masonry primer
- On smooth stucco: 7.81 gallons; on heavy texture: 13.02 gallons
- Elastomeric, two coats at 80 sq ft per gallon: 24.41 gallons
From the stucco and masonry paint calculator.
Elastomeric is meant to go on thick, typically 15 to 20 wet mils a coat, which is why its coverage is a third or less of ordinary paint’s. Thinning it or spreading it further defeats its purpose of bridging cracks. Use it where stucco has hairline cracking; on sound stucco, acrylic masonry paint is cheaper and breathes better.
Brick and block
Painting brick is a one-way decision: once painted, brick is very hard to return to bare, and paint can trap moisture in some older, softer bricks. If the aim is water protection rather than colour, a clear penetrating sealer may be the better choice; the masonry sealer calculator covers it.
Concrete block is so porous and pitted that it needs a block filler first, a thick primer that fills the pores so the finish coat can form a continuous film.
640 sq ft of concrete block
- Block filler at 100 sq ft per gallon, one coat: 6.72 gallons, buy a 5-gallon pail, a gallon and three quarts
- Split-face block takes more, at about 70 sq ft per gallon
From the block filler calculator. The finish coats then go on at something like masonry paint’s normal coverage.
Fibre cement, vinyl and metal
Fibre cement siding is usually factory-primed or factory-finished and paints well with an exterior acrylic; treat smooth panels as a factor of 1.0 and lap boards as 1.1.
Vinyl can be painted with paints formulated for vinyl, but dark colours can make it warp in the sun, because vinyl softens with heat. Check the siding maker’s guidance and use a paint brand’s vinyl-safe colour range.
Aluminium and steel siding need cleaning of chalk and a primer suited to the metal where it is bare; then they paint like smooth siding.
Coats on exteriors
Bare and weathered surfaces take a primer and two finish coats. Sound paint going to a similar colour can take one good coat on a sheltered wall; on the south and west faces, which take the most sun, two coats last noticeably longer. The coats calculator applies the colour rules, and paint coverage explained covers why the first coat on porous surfaces goes less far.
Prep comes first
Every siding type fails for the same reason when it fails early: paint applied over dirt, chalk, mildew or loose old paint. Wash the walls with a house cleaner and a soft brush or a low-pressure washer, and rinse. Rub a hand on the old paint: if it comes away white, the surface is chalking and needs washing until it does not, or a primer that binds chalk. Scrape loose paint, sand the edges, spot-prime bare wood, and caulk open joints around trim and windows. On a house built before 1978, the old paint may contain lead; see the disclaimer on this site and the US EPA’s guidance before sanding or scraping. Prep is usually more of the job than the painting, and the part that decides how long the paint lasts.
Weather for painting
Most exterior latex paints need air and surface temperatures above about 50 °F during application and for some hours after, with no rain forecast and no dew settling on the wall before the paint has set. Some products are labelled for application down to 35 °F; check the can. Avoid painting a wall in full, hot sun, which dries the paint too fast and leaves lap marks. Follow the shade round the house through the day.
Common mistakes
- Using the smooth-surface coverage on stucco or brick. Plan with the factor or the product’s textured figure.
- Painting new stucco too early. Let it cure, then prime it.
- Spreading elastomeric thin. It is meant to be thick.
- Skipping block filler on concrete block. The finish coat pinholes and fails.
- Painting cedar without a stain-blocking primer. Tannin streaks show through light colours.
- Dark colours on vinyl. Use vinyl-safe colours or expect warping.
For how to measure the house in the first place, see measuring a house exterior.